Selection of magnetic materials suitable for use in relays, ringers, and electro-acoustic transducers such as loudspeakers and telephone receivers is characteristically based on magnetic properties such as high magnetic coercivity, remanence, and energy product. Among established alloys having suitable magnetic properties are Fe-Al-Ni, Fe-Al-Ni-Co, Cu-Ni-Fe, and Cu-Ni-Co alloys; more recently, alloys containing Fe, Cr, and Co have been investigated with regard to potential suitability in the manufacture of permanent magnets.
Specifically, Fe-Al-Ni, Fe-Al-Ni-Co, Cu-Ni-Fe and Cu-Ni-Co alloys and alloy processing are disclosed, e.g., in the books by Richard M. Bozorth, "Ferromagnetism," Van Nostrand, New York 1951, pages 385-405, and by D. Hadfield (ed.), "Permanent Magnets and Magnetism," John Wiley and Sons, New York 1962, pages 132-155 and page 164. Certain ternary Fe-Cr-Co alloys are disclosed in the paper by H. Kaneko et al., "New Ductile Permanent Magnet of Fe-Cr-Co System," AIP Conference Proceedings, No. 5, 1972, p. 1088 and in U.S. Pat. No. 3,806,336, "Magnetic Alloys" (issued to H. Kaneko et al. on Apr. 23, 1974). Quaternary alloys containing ferrite-forming elements such as, e.g., Ti, Al, Si, Nb, or Ta in addition to Fe, Cr, and Co are disclosed in U.S. Pat. No. 3,954,519, "Iron-Chromium-Cobalt Spinodal Decomposition Type Magnetic Alloy Comprising Niobium and/or Tantalum" (issued to K. Inoue on May 4, 1976), U.S. Pat. No. 3,989,556, "Semihard Magnetic Alloy and a Process for the Production Thereof" (issued to M. Iwata et al. on Nov. 2, 1976), U.S. Pat. No. 3,982,972, "Semihard Magnetic Alloy and a Process for the Production Thereof" (issued to M. Iwata et al. on Sept. 28, 1976), U.S. Pat. No. 4,075,437, "Composition, Processing, and Devices Including Magnetic Alloy" (issued to G. Y. Chin et al. on Feb. 21, 1976), the paper by G. Y. Chin et al., "New Ductile Cr-Co-Fe Permanent Magnet Alloys for Telephone Receiver Applications", Journal Applied Physics, Volume 49, No. 3, 1978, p. 2046, the paper by H. Kaneko et al., "Effect of V and V+Ti Additions on the Structure and Properties of Fe-Cr-Co Ductile Magnet Alloys," IEEE Trans. Mag., Volume MAG-12, No. 6, 1976, p. 977, and the paper by H. Kaneko et al., "Fe-Cr-Co Ductile Magnet with (BH).sub.max =8 MGOe," AIP Conf. Proc., No. 29, 1976, p. 620. Further development of Fe-Cr-Co alloys and alloy processing is disclosed in pending U.S. patent applications Ser. Nos. 924,137 and 924,138. Fe-Cr-Co alloys containing rare earth additions are disclosed in U.S. Pat. No. 4,120,704, issued Oct. 17, 1978 in the name of Richard L. Anderson.
Highly magnetic alloys typically have a decomposed multi-phase structure comprising at least one strongly magnetic phase and one or several nonmagnetic or weakly magnetic phases. For example, in the case of Fe-Cr-Co alloys, a highly magnetic phase alpha.sub.1 is rich in Fe and Co and a weakly magnetic phase alpha.sub.2 is rich in Cr. Similarly, in the case of Fe-Ni-Al alloys, a phase alpha.sub.1 rich in Fe is highly magnetic and a phase alpha.sub.2 rich in Ni and Al is weakly magnetic. Different notation is customarily used in the case of Cu-Ni-Fe alloys, where a strongly magnetic phase rich in Fe and Ni is designated gamma.sub.2 and a weakly magnetic phase rich in Cu by gamma.sub.1. It is generally postulated that achievement of high coercivity in such alloys is fostered by the development of a so-called spinodal structure, i.e., a submicroscopically fine decomposed two-phase structure in which a highly magnetic phase is interspersed with a weakly magnetic phase. Development of such postulated spinodal structure is typically the result of an aging heat treatment of the alloy at temperatures which correspond to an equilibrium two-phase state and which are at or below a so-called spinodal temperature.
It has been realized that magnetic properties of Fe-Cr-Co alloys can be enhanced by producing anisotropy in a decomposed two-phase structure by an aging heat treatment in the presence of a magnetic field. As disclosed in the paper by J. W. Cahn, "Magnetic Aging of Spinodal Activity," Journal Applied Physics, Volume 34, No. 12 (1963), p. 3581, such treatment is characteristically applied to alloys whose spinodal temperature lies close to the Curie temperature. Such treatment may be less applicable, however, to alloys whose spinodal temperature is significantly higher than the Curie temperature as is the case, e.g., for cobalt-free Fe-Al-Ni, low-cobalt Fe-Al-Ni-Co, Cu-Ni-Fe, low-cobalt Fe-Cr-Co alloys, and other alloys as disclosed by F. E. Luborsky, "Permanent Magnets in Use Today," Journal Applied Physics, Volume 37, No. 3, page 1091 (1966). Since Co is relatively expensive as a component in magnetic alloys, means for producing strong anisotropy in low-Co and Co-free alloys are commercially desirable.